1) Field of the Invention
The present invention relates, in general, to amplification of signal light in optical networking and in particular, to a semiconductor laser device that does not require a temperature control module. Particularly, the present invention relates to an erbium doped optical fiber amplifier (EDFA) that produces a stable amplification gain of signal light. Further, the invention relates to a semiconductor laser device that has two stripe structures, a semiconductor laser module and an optical fiber amplifier that is compact and easy to manufacture, and achieves high output power while reducing stimulated Brillouin scattering in Raman amplifier and the degree of polarization of the laser beam emitted from the semiconductor laser device.
2) Description of the Related Art
With the recent advancements in the field of optical networking which has internet technology at its core, an optical fiber amplifier embedding between the each span of the transmitting optical fibers is widely practiced to enhance the transmission of light signals over longer distances. The optical fiber amplifier revives weak light signals by amplifying them. For example, if an exciting light having a central wavelength of around 980 nm or 1480 nm is injected into an erbium-doped fiber (EDF), the signal lights of 1550 nm range are amplified. A wavelength of around 1550 nm is generally preferred for transmission as it has very little transmission loss as compared to other wavelengths.
FIG. 78 is a graph that shows the relationship between the exciting light wavelength and the absorption coefficient of EDF at 980 nm range. It is evident from the graph that the absorption coefficient is the maximum at a wavelength of 978 nm and therefore the optical gain of Erbium doped fiber amplifier (EDFA) is the maximum at this wavelength. Hence, a high-gain optical fiber amplifier can be realized if EDFA is excited by the exciting light having a wavelength of 978 nm.
However, there are some requirements in the selection of excitation light source for the currently available optical fiber amplifiers requires caution. Fabry-Perot semiconductor laser devices are being widely used as the conventional excitation light source. However, the oscillation wavelength of the laser beam emitted from by a Fabry-Perot semiconductor laser device depends greatly on the temperature of the active layer and the current injected (injected current) in the active layer. The temperature of the active layer, because it emits the laser beam, rises as injection current into the active layer increases and the ambient temperature increases when oscillation occurs continuously.
The change in the oscillation wavelength of the laser due to the temperature change has a direct effect on the absorption coefficient of the EDF. The absorption coefficient of the EDF also changes, as can be seen in FIG. 78, with a change in the oscillation wavelength of the exciting laser. Hence it is very difficult to obtain a constant amplification gain independent on the temperature change of exciting laser in the EDFA.
Particularly, in the Fabry-Perot-type semiconductor laser device, if the temperature of the active layer rises by 1° C., the oscillation wavelength shifts by around 0.4 nm towards the longer wavelength side. Assuming that a semiconductor laser device oscillating at 978 nm at a certain temperature is the excitation light source, and that the temperature of the active layer has risen by 20° C., the oscillation wavelength of the emitted laser beam in this case will be 986 nm. As a result, the absorption coefficient of the EDF will decrease from 5 dB to 3 dB, as can be seen in FIG. 78. This indicates that it is not possible to obtain a constant amplification gain in EDFA independent of the oscillating wavelength change due to the temperature change of active layer of the laser.
A technology for obtaining exciting light of constant wavelength by mounting a semiconductor laser device on top of a temperature-adjusting module (hereinafter, referred to as prior art 1) is known. The prior art 1 discloses a semiconductor laser module that has a semiconductor laser device mounted on a thermo-electronic cooler and a light filter that allows laser beam having only a specific wavelength to pass. In the prior art 1, exciting light of constant wavelength is obtained by maintaining the intensity of the laser beam by monitoring the light form the reflection facet of the semiconductor laser device and controlling the temperature of the active layer of the semiconductor laser device by controlling a sensor (thermistor) temperature. For details, see the patent literature 1 mentioned below.
A technology that does not use a thermo-electronic cooler in the semiconductor laser module (hereinafter, referred to as prior art 2) is also known. This technology is described in detail with reference to FIG. 79. There are provided a first Fabry-Perot semiconductor laser module 201 that emits a laser beam of a first oscillation wavelength λ1 at a specific basic temperature, and a second Fabry-Perot semiconductor laser module 202 that emits a laser beam of a second oscillation wavelength λ2 at the same basic temperature. The laser beams emitted from the first Fabry-Perot semiconductor laser module 201 and the second Fabry-Perot semiconductor laser module 202 are multiplexed in a 50/50 coupler 203. The laser beam with the first oscillation wavelength λ1 and the laser beam with the second oscillation wavelength λ2 is split into two and one of them is multiplexed with a signal light 204 in WDM coupler 206 and subsequently passed into the amplification optical fiber 207. It is assumed here that the amplification optical fiber is the EDF and the amplification of signal light is achieved by combining the signal light with two types of laser beams, that is, the laser beam with the oscillation wavelength λ1 and the laser beam with the oscillation wavelength λ2. It is also assumed that, the Fabry-Perot semiconductor laser modules 201, 202 do not have an electronic cooling device and hence the temperature of the active layers cannot be controlled.
The mechanism of amplification in the prior art 2 is explained next with reference to FIG. 78. When amplifying signal light by using two laser beams of different wavelengths λ1 and λ2, the absorption coefficient of the EDF will be α+β where a is the absorption coefficient for wavelength λ1 and β is the absorption coefficient for wavelength λ2.
As explained already, the temperature of the Fabry-Perot semiconductor laser modules 201, 202 increases, and therefore, the wavelengths shift towards the longer wavelength side. Let us assume that the temperatures of the Fabry-Perot semiconductor laser modules 201, 202 have risen by ΔT(K) and the respective wavelengths have increased by Δλ(=0.4×ΔT)(nm).
Let us also suppose that the respective absorption coefficients α and β have changed to α′ and β′ because of the shift in the wavelengths. As can be seen from the graph in FIG. 78, the wavelength at which the absorption coefficient is the maximum is λ0. If the oscillation wavelengths λ1 and λ2 of the Fabry-Perot semiconductor laser modules 201 and 202 are set in such a way that λ1<λ0<λ2, then α′>α and β′>β. However, if the absolute values of the amount of change of absorption coefficients, α(T) and β(T), are identical, then α+β=α′+β′. As a result, the absorption coefficient of the EDF would be constant irrespective of the temperature of the active layer. Therefore, by setting λ1 and λ2 in such a way that the sum of absorption coefficients α(T)+β(T) is always constant, a constant amplification gain of the signal light 204 can be obtained for EDFA without the use of a temperature adjusting module or a wavelength monitoring section. For detail explanation see non-patent literature 2 mentioned below.
Patent literature 1: Japanese Patent Laid-Open Publication No. H10-79551.
Non-patent literature 2: P. Vavassori, R. Sotgiu, “New EDFA pumping scheme insensitive to 980 nm diode lasers temperature variation”, OtuB3, 2001 Technical Digest on Optical Amplifiers and Their Applications, July 2001, Stresa, Italy.
Conventionally in an optical fiber amplifier using Raman amplification scheme, for example, it is known that a plurality of semiconductor laser devices are used as excitation light sources so that a high-output excitation light source is realized and a high-Raman gain optical fiber amplifier can be realized by using it. In the Raman amplification, since the signal light is amplified in a state that polarization directions of the signal light and the exciting light correspond with each other, it is necessary to reduce the influence of a deviation of a plane of polarization between the signal light and the exciting light as much as possible. For this reason, polarization of the exciting light is nullified (depolarization) so that the degree of polarization (DOP) is reduced.
FIG. 80 is a block diagram that shows one example of a structure of a conventional Raman amplifier used in a WDM communication system. As shown in FIG. 80, the conventional Raman amplifier includes semiconductor laser modules 682a through 682d. Each semiconductor laser module includes a Fabry-Perot type semiconductor light emission element and a fiber grating. That is, the semiconductor laser modules 682a through 682d include Fabry-Perot type semiconductor light emission elements 680a through 680d and fiber gratings 681a through 681d. The semiconductor laser modules 682a and 682b output laser beams, which are the excitation light source, to a polarization beam combiner 661a. The semiconductor laser modules 682c and 682d output laser beams, which are the excitation light source, to a polarization beam combiner 661b. The semiconductor laser modules 682a and 682b emit laser beams of same wavelengths and the polarization beam combiner 661a combines (multiplexes) these two laser beams to obtain light having perpendicularly polarized planes. Similarly, the respective semiconductor laser modules 682c and 682d emit laser beams of same wavelength and the polarization beam combiner 661b combines these two laser beams to obtain light having perpendicularly polarized planes. The polarization beam combiners 661a and 661b output the polarization multiplexed laser beams to a WDM coupler 662. The laser beams output from the polarization beam combiners 661a and 661b have different wavelengths.
The WDM coupler 662 multiplexes the laser beams output from the polarization beam combiner 661a and 661b via an isolator 660, and outputs the multiplexed laser beam as an exciting light to an amplification fiber 664 via a WDM coupler 665. A signal light to be amplified is also input into the amplification fiber 664 from a signal light input fiber 669 via an isolator 663. The signal light is coupled with the exciting light and the signal light is thus Raman-amplified.
When the laser beams to be polarization-combined are emitted from different semiconductor elements, the process of fabrication of the optical fiber amplifier become complicated and the optical fiber amplifier becomes bulky. To overcome this drawback, a technique of fabricating the Raman amplifier using a semiconductor laser device having two light emission areas at one chip is proposed. As a result of this technique, the process of fabrication of the optical fiber amplifier is simplified, and since a plurality of stripes are mounted on the same substrate, the semiconductor laser device itself can be miniaturized.
The semiconductor module used in the prior art 1 has a complex structure and function and besides is not cost-effective. Particularly, since the thermo-electronic cooler consume electricity to cool the semiconductor laser device, total power consumption of semiconductor laser module also goes up.
Complex structure translates to uneconomical production time and cost. Also, there is a high probability of breakdown. An optical fiber amplifier is expected have a long and trouble-free service life.
There are problems with the prior art 2 as well. If the value of λ1 is set closer to of λ0, the oscillation wavelength λ1+Δλ exceeds λ0. When this happens, the absorption coefficient α(T) decreases and therefore is unable to counterbalance the reduction in absorption coefficient β(T). The result is inability to maintain a constant gain.
To maintain a constant gain, it is necessary to set a small value for wavelength λ1 in order to make the absorption coefficient α(T) as a monotone increasing function. The absorption coefficient (α(T)+β(T)) would decrease and so would the gain during signal light amplitude. The prior art 1 is superior to the prior art 2 from gain point of view since in the prior art 1 the absorption coefficient can be maximized by properly controlling the temperature of the active layer and setting the value of the oscillation wavelength to λ0.
Also, in the prior art 2, the gain is not constant because the temperature of the active layer changes. Since the Fabry-Perot type semiconductor laser device is used as an excitation light source in the prior art 2, the range of fluctuation of wavelength with respect to temperature variation is large. Therefore, it is difficult to determine wavelengths λ1 and λ2 from such a wide spectrum of the absorption coefficient of EDF such that the total of absorption coefficient α(T)+β(T) is constant. For instance, it can be seen that even in the case represented by FIG. 78, the rate of increase of α(T) is not equal to the rate of increase of β(T) and hence the sum of absorption coefficient α(T)+β(T) decreases as the temperature increases.
On the other hand, when a semiconductor laser element having a structure of plural stripes, particularly a structure of two stripes (W-stripe structure) is used in the Raman amplifier, a new problem arises. In other words, when the semiconductor laser element having the W-stripe structure is used, DOP is not reduced as compared with when separated semiconductor elements are polarization-combined.
When DOP is not reduced, the degree of polarization of laser beam from the excitation light source is not nullified. Since the amplification gain in the Raman amplifier is determined by the intensity of the exciting laser beam component having the same polarization as that of the signal light, when the laser beam from the excitation light source is polarized to a specified direction, the amplification gain in the Raman amplifier changes due to the polarization direction of the signal light. In other words, since a stable amplification gain cannot be obtained, the semiconductor laser element having the W-stripe structure is not suitable has a problem for use as an excitation light source of the Raman amplifier.
The cause of the difference in the reduction of DOP is as follows. In the independent semiconductor laser elements as shown in FIG. 80, even when the oscillation wavelengths of each of the semiconductor laser elements are set to be uniform at the design stage, the oscillation wavelengths are not exactly equal, although the difference is very small, due to scattering in actual production. On the contrary, in the W-stripe structure, at the actual production step, a cleavage to allow for an epitaxial growth and to form a reflection end surface is normally formed in exactly identically for each stripe. Therefore, the structures of the stripes are exactly identical, and thus the oscillation wavelengths are the same. The difference in the reduction of DOP occurs due to such a difference in the structures.
Therefore, in order to reduce DOP in the semiconductor laser element having the W-stripe structure, for example, end faces may be formed separately by cleavage so that resonator lengths of the respective stripes are different. However, since the distance between the stripes in the semiconductor laser element is only within about a few hundred μm, it is neither easy nor realistic to carry out cleavage separately.
On the contrary, as shown in FIG. 80, in the structure that the independent semiconductor laser elements are polarization-combined, DOP can be reduced, and the amplification gain which is stabilized regardless of the polarization direction of the signal light can be obtained as a Raman gain. However, the production process of the Raman amplifier having the structure shown in FIG. 80 is complex and further it is difficult to miniaturize the entire device.
On the other hand, when semiconductor laser elements are used as the excitation light sources of a distribution type amplifier such as the Raman amplifier, it is preferable that the exciting light output power is increased in order to increase the Raman amplification gain. However, when its peak value is high, stimulated Brillouin scattering occurs, and this results in noise. It is essential to increase the threshold value at which the stimulated Brillouin scattering occurs.
Patent literature 2: Japanese Patent Laid-Open Publication No. H5-145194.